V2CTranscoder is a middleware tool that aggregates and groups CAN data frames. It is designed to run on a device connected to a CAN bus that sends aggregated frames to a remote server or stores them locally.
The transcoder takes CAN frames as input, aggregates their values, and encodes them back into CAN frames. The resulting frames are more suitable for sending over the network because of their reduced frequency. Frames received by the server will appear identical to frames sent over CAN, making the tool transparent from the server's perspective.
The tool can be configured to send different messages at different frequencies or to send the average value of a signal over a time window.
To use V2CTranscoder, initialize v2c_transcoder and call its transcode(t, frame) method with frames read from the CAN socket.
can_frame frame = read_frame();
auto fp = transcoder.transcode(std::chrono::system_clock::now(), frame);The result fp is of the type can::frame_packet, defined in frame_packet.h, and is a serialized list of CAN frames with timestamp information.
The frame packet is not sent unless more than V2CTxTime milliseconds have passed since the last transmission.
A frame_packet can be iterated to get raw frames with their timestamps:
for (const auto& [ts, frame] : fp) {
// use frame.can_id, frame.data
}frame is of type can_frame, defined in can_kernel.h, and ts is a std::chrono::system_clock::timepoint.
The class frame_packet provides the following methods for accessing its raw data, to send the bytes over the network:
const uint8_t* data_begin() const {
return _buff.data();
}
const uint8_t* data_end() const {
return _buff.data() + _buff.size();
}
std::vector<uint8_t> release() {
return std::move(_buff);
}On the server side, the same frame_packet can be recreated directly from a byte buffer as such:
std::vector<uint8_t> buffer(buffer_size);
read_socket.read_some(boost::asio::buffer(buffer));
can::frame_packet fp(std::move(buffer));All V2C configuration is in the DBC input file, in DBC syntax, using attributes and environment variables.
EV_ V2CTxTime: 0 [0|60000] "ms" 2000 1 DUMMY_NODE_VECTOR1 V2C;The environment variable V2CTxTime is used to specify the time between two transmissions.
Each transmission contains all aggregated frames since the last transmission in a frame_packet.
Groups are defined by a set of message IDs and a frequency.
The messages of a group are treated as a single unit: If a single message of a group does not arrive over the CAN, the entire group is not sent.
This ensures that no outdated data is sent.
To define a new group, add a new environment variable with the suffix GroupTxFreq.
The value of this variable determines the sending period of this group in milliseconds. (600ms, 500ms)
The sending frequency (2000ms) does not have to match message aggregation frequencies.
EV_ GPSGroupTxFreq: 0 [0|60000] "ms" 600 11 DUMMY_NODE_VECTOR1 V2C;
EV_ EnergyGroupTxFreq: 0 [0|60000] "ms" 500 13 DUMMY_NODE_VECTOR1 V2C;For signals that change slowly over time, such as from temperature sensors, it makes sense to send them less frequently.
To add a message to a group, set its "TxGroupFreq" to the group name:
BA_ "TxGroupFreq" BO_ 2 "GPSGroupTxFreq";
BA_ "TxGroupFreq" BO_ 3 "GPSGroupTxFreq";
BA_ "TxGroupFreq" BO_ 5 "GPSGroupTxFreq";
BA_ "TxGroupFreq" BO_ 7 "GPSGroupTxFreq";
BA_ "TxGroupFreq" BO_ 4 "EnergyGroupTxFreq";
BA_ "TxGroupFreq" BO_ 6 "EnergyGroupTxFreq";The above attributes assign messages GPSLatLong, GPSAltitude, GPSSpeed, and PowerState to the GPSGroupTxFreq group,
and the messages SOC and BatteryCurrent to the EnergyGroupTxFreq group.
Two aggregators are supported:
LASTis used to send the most recent value of the signal. This is the default.AVGis used to send the average value of the signal.
To set a signal's aggregation type, set its "AggType" attribute to either LAST or AVG:
BA_ "AggType" SG_ 2 GPSAccuracy "LAST";
BA_ "AggType" SG_ 2 GPSLongitude "LAST";
BA_ "AggType" SG_ 2 GPSLatitude "LAST";
BA_ "AggType" SG_ 3 GPSAltitude "LAST";
BA_ "AggType" SG_ 5 GPSSpeed "LAST";
BA_ "AggType" SG_ 7 PowerState "LAST";
BA_ "AggType" SG_ 4 SOCavg "LAST";
BA_ "AggType" SG_ 6 RawBattCurrent "AVG";
BA_ "AggType" SG_ 6 SmoothBattCurrent "AVG";
BA_ "AggType" SG_ 6 BattVoltage "AVG";The aggregation is done in time windows that match the group's sending period. No single CAN measurement is sent or aggregated more than once.
For example.dbc described above, transcoder.transcode(ts, frame) returns a new frame_packet every 2000 ms
containing up to four GPS groups and up to four Energy groups, in chronological order.
Each group contains an aggregated can_frame for each message in that group.
To filter out a signal, simply do not include it in any group. Only signals that are part of a group are aggregated and appended to the resulting frame_packet.
Messages are often multiplexed by a switch that determines which signal is currently active:
BO_ 10 MuxedMessage: 4 DUMMY_NODE_VECTOR1
SG_ MuxSwitch M : 0|8@1+ (1,0) [0|255] "" Receiver
SG_ Signal1 m1 : 8|8@1+ (2,0) [0|510] "" Receiver
SG_ Signal2 m2 : 8|8@1+ (0.5,0) [0|127.5] "" Receiver
SG_ Signal3 m3 : 8|8@1+ (1,0) [0|255] "" Receiver
Sg_ Signal4 : 16|16@1+ (0.05,0) [-1000|1000] "" ReceiverSignals 1 to 3 all occupy the same bit range, and are multiplexed by MuxSwitch.
This means that if we want to calculate the average of all three signals separately, their average values would not fit into a single can_frame.
To solve this problem, the final frame_packet contains a separate can_frame for each multiplexed signal value.
For the above message, three can_frames are generated, each encoded with one of the multiplexed signals (and the corresponding mux switch value).
Signal4 will be aggregated as usual, from every input can_frame regardless of its mux switch value.
To avoid duplication on the server side, Signal4 is encoded only in the first can_frame,
and the other frames are marked as duplicates by setting their frame.__res0 byte to 1.
This is a reserved padding byte not used for anything else.
The transcoder only has to use six different tag_invokes for DBC parsing and data structure initialization.
The short parsing functions can be found at the end of v2c_transcoder.h.